1. Field of the Invention
The present invention relates to a digital signal transmitting apparatus and digital signal transmitting method for performing asynchronous transmission of a digital signal.
2. Description of the Related Art
Japanese Patent No. 3344530 has disclosed a digital signal transmitting apparatus including: a modulator which modulates a data clock and digital signals of multiple channels synchronized with the data clock and thereby produces a transmit digital signal; a single transmission line used to transmit the transmit digital signal; and a demodulator which demodulates the modulated transmit digital signal to produce the data clock and the digital signals of the multiple channels. The demodulator includes a PLL circuit which synchronizes the data clock with the transmit digital signal. In the digital signal transmitting apparatus described in the Patent Document, the duty factor (pulse width) of the transmit digital signal is varied according to a combination of the binary levels of digital signals of multiple channels, thereby allowing data discrimination.
However, the above described digital signal transmitting apparatus requires a PLL circuit, so the circuit size increases; further, since it takes a lock time for the PLL circuit at the time of startup, the start-up time is prolonged. Meanwhile, there is a system which does not use a PLL circuit and is provided with independent clocks in the modulator and demodulator so that asynchronous transmission of digital signals is performed. In this case, a difference of a predetermined value or more between the clock of the modulator and the clock of the demodulator causes erroneous transmission, so considerations must be given to prevent the erroneous transmission.
Thus an object of the present invention is to provide a digital signal transmitting apparatus and digital signal transmitting method of an asynchronous type capable of suppressing erroneous transmission caused by such clock variation.